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based on an earlier version of the Wolfram Language.
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TraceScan

TraceScan[f, expr]
applies f to all expressions used in the evaluation of expr.
TraceScan[f, expr, form]
includes only those expressions which match form.
TraceScan[f, expr, s]
includes all evaluations which use transformation rules associated with the symbol s.
TraceScan[f, expr, form, fp]
applies f before evaluation and fp after evaluation to expressions used in the evaluation of expr.
  • All expressions are wrapped in HoldForm to prevent evaluation before f or fp are applied to them.
  • The function fp is given as arguments both the form before evaluation and the form after evaluation.
  • TraceScan[f, expr] returns the result of evaluating expr.
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